(a) Field of the Invention
The present invention relates to an aplanatic single lens of which aberrations are corrected to the degree that the diameter of the circle of confusion thereof is decided approximately by diffraction of light and, more particularly, to a large-aperture single lens with aspherical surfaces of which both surfaces are formed as aspherical surfaces having positive refractive powers.
These types of lenses have a large field of application as various kinds of condensers and, recently, they are attracting attention particularly as the pickup objectives for optical video disks.
(b) Description of the Prior Art
For pickup lenses to be used with video disk reproducing devices employing optical pickups or the like, it is essential that N.A. is 0.4 to 0.5 and aberrations are corrected to the degree that the residual axial aberration comes within the Rayleigh limit, because a resolving power about 1,000 lines/mm is required.
When one tries to arrange such a lens system by glass lenses with spherical surfaces, multiple lenses should be combined as in the case of microscope objectives. Consequently, the overall length of the lens becomes long and it is difficult to make the lens compact and light in weight. Besides, adjustment of eccentricity etc. between multiple lenses becomes complicated and high-accuracy machining is indispensable for the lens tube. Therefore, it is impossible to reduce the cost of the lens due to te above-mentioned reasons. Moreover, as the number of lenses constituting the lens system becomes large, the loss of intensity of light becomes large due to reflection by inner surfaces and absorption, and flare causes unfavourable influence on the S/N ratio of reproducing output.
Recently, as the tracking method for the video disks, the method of directly driving the objective has been adopted instead of the method of swinging a mirror like a galvanometer and, therefore, it is necessary to make the lens compact and light in weight. In this case, however, the axial focal point of the optical system is always utilized and, consequently, it is not necessary to give as much consideration to offaxial aberrations. To meet the above-mentioned requirement, it has been suggested to use a single lens with an aspherical surface (for example, in Japanese published unexamined patent application No. 156945/75). This kind of single lens can be made extremely light in weight by manufacturing it with a plastic material. However, the above-mentioned known lens has disadvantages as described below. That is, as said lens is designed to have the focal length about 7.5 mm, it is impossible to obtain a sufficient working distance when the focal length is made about 4 to 5 mm. Especially, as the second surface is made concave toward the image surface, the working distance on the second surface side becomes short compared with the focal length.